Variations in the pathophysiological responses of individuals following injuries, infections or other biological or environmental challenges are in part driven by genetic differences. It is also accepted that gender modulates pathophysiological responses. Although it is still controversial, females seem to show better outcome than males following injuries or infections. The current notion is that sex hormones are the underlying causes of gender-dimorphic pathology; however this notion is challenged by the fact that pre-pubertal children as well as older individuals manifest gender-differences in disease outcome and experimental sex hormone replacements only partially diminish sex-based differences in animal models. These facts indicate that factors other than sex hormones contribute to gender-dimorphic outcomes in disease and health. The proposed human investigations will test the hypothesis that common X-linked genetic polymorphisms combined with gender-specific regulation of X-linked immune genes are important contributors to gender-based outcome differences after trauma. This concept is based on the fact that females carry the inherently polymorphic maternal and paternal X chromosomes (ChrXs) whereas males carry only the maternal ChrX. Furthermore, females display cellular mosaicism for the expression of polymorphic X-linked genes due to random ChrX inactivation. Therefore, ChrX cellular mosaicism in females is expected to increase cellular variability compared to males, thereby improving cellular responses to dynamically changing pathophysiological conditions following injuries. The primary focus of the project is the X-linked IRAK1, which is a critical mediator of TLR-dependent immune cell activation. Human IRAK1 polymorphism is common (20-40% frequency) and the variant IRAK1 has robust clinical effects including doubling the frequency of shock, pneumonia and mortality in septic and trauma patients. The first aim of the project will test whether variant IRAK1 is associated with worsened outcome in severely injured male and female trauma patients and whether cellular mosaicism for variant IRAK1 expression in females is beneficial during the trauma course. The study will also test the effects of variant IRAK1 or IRAK1-mosaicism on white blood cell activation and ChrX skewing following major injuries. We will also use blood from healthy individuals to test whether variant IRAK1 mosaicism is associated with increased functional variability of neutrophils, monocytes, T and B cells in females and how alternative IRAK1 splice variants and their allelic variants affect myeloid and lymphoid cell responses. Lastly, we will test associations between other X-linked immuno-modulatory genes and worsened clinical outcome or altered white blood cell responses. Due to the X-linked inheritance pattern, the biological impact of X-linked polymorphisms is always greater on males than females. This fact combined with female cellular ChrX mosaicism with an expected increase in cellular variability ensures that gender-based outcome differences have an X-linked genetic component. Results from these studies will help explain current controversies in gender-biased outcome differences and will help in predicting outcome and treating the severely ill.